1. Field of the Invention
The invention relates to a method and apparatus for locating the neuronal sources of magnetic signals emanating from the brain under normal or pathological conditions. Neuromagnetic signals seen under normal conditions might result from evoked responses from visual, auditory, somatic, or other type stimulus. Normal signals may also be used to study a patient's brain rhythms, such as alpha, beta, etc. Neuromagnetic signals seen under pathological conditions might be associated with epilepsy or some other disease.
2. Description of the Contemporary and/or Prior Art
Locating neuronal sources of magnetic signals emanating from the brain under normal or pathological conditions is of significance in clinical diagnostics. Neuromagnetic signals seen under normal conditions might measure evoked responses from visual, auditory, somatic, or other type of stimulus. Normal signals might also be used to study brain rhythms, such as alpha and beta brain rhythms. Neuromagnetic signals seen under pathological conditions might be associated with epilepsy or some other disease.
The neuromagnetic signals that originate from sources in the brain can be modeled by current dipoles. The current dipole causes volume currents, J, to flow in the brain that, at the brain surface, cause potential differences, V, at the scalp, which are signals measured by the EEG (electroencephalograph). A magnetic field, B, associated with the current dipole is also generated and is the neuromagnetic field measured by the MEG (magnetoencephalograph). The neuromagnetic field has a field spatial pattern giving contours of constant field (plus contours for the B field vector coming out, minus contours for the B field vector going in). The magnitude of the field diminishes as the contour radius increases.
The MEG has theoretical advantages over the EEG. The vector B gives directional information about the source orientation. The neuromagnetic field B is not distorted by the brain, which has the same permeability as air. The MEG is an absolute measure of source strength not measured with respect to a reference, as is the EEG. The MEG is not affected by bad electrode contact or tissue artifacts, as is the EEG. The MEG does not need to touch the head, as in the EEG.
The prior art apparatus and method requires movement of the MEG to a plurality of positions over the skull of the patient. At each position data is collected. A contour map is then drawn from the data and the dipole location is found along a line drawn connecting the plus peak contour with the minus peak contour at a depth using algebraic theory for a dipole in a sphere. Several problems occur with this prior art method and apparatus for localization. First, it takes hours to accomplish these measurements and the neuronal source is assumed to be temporally and spatially constant. Second, the mathematical model used to attain source depth is valid only for a sphere, which the head is not. Third, the accuracy of localization is dependent on the signal-to-noise ratio, number of measurement points, and dipole orientation relative to the MEG measurement axis. Fourth, the algorithms used to fit theory to contour maps are computationally burdensome.
Moving the MEG from station to station causes three major problems. First, because of their extreme sensitivity and vector measurement capability, the prior art MEG devices can vibrate and give false signals due to their motion in the earth's static magnetic field. At each measurement station the experimental vibration spectrum is different. Second, present MEG devices are cryogenically cooled with liquid helium (i.e., they are superconducting magnetometers). Movement of these sensors from station to station around the head to generate contour maps causes inaccuracies in calibration due to tilt and changes in helium levels. Third, at each new station or measurement point the magnetic balance or magnetic nulling of device noise with internal magnetic trim tabs changes, giving rise to another calibration error.
Wynn et al, in an article entitled "Advanced superconducting gradiometer/magnetometer arrays and a novel signal processing technique", IEEE Trans. Mag., Vol. 2, pg. 701, 1975, showed that a five-axis gradiometer magnetometer can provide a three-dimensional localization and track of a large source strength, static, ferrous magnetic dipole at ranges from tens of feet to thousands of feet, according to the dipole source strength and magnetometer sensitivity. Their device used magnetic sensors spaced many inches apart, not intended or designed for neuromagnetic measurements. They also showed that a simpler three-axis device could provide two-dimensional location and track of larger dipole objects. In summary, neuromagnetic localization with prior art MEG devices uses single point measurements that provide data to create a spatial magnetic contour map as the device is moved from station to station. These MEG measurements are inadequate for many reasons:
a. Because of their extreme sensitivity and vector measurement capability, the prior art MEG devices can vibrate and give false signals due to their motion in the earth's static magnetic field. At each measurement station the experimental vibration spectrum is different. PA1 b. Prior art MEG devices are cryogenically cooled with liquid helium (e.g., they are superconducting magnetometers). Movement of these sensors from station to station around the head to generate contour maps causes inaccuracies in calibration due to tilt and changes in helium levels. PA1 c. At each new station or measurement point the magnetic balance or magnetic nulling of device noise with internal magnetic trim tabs changes, giving rise to another calibration error. PA1 d. It can take several hours to complete the data matrix of measurement positions and the neuronal source may not remain the same over that time period for valid localization. PA1 e. The localization accuracy under this method is dependent on fitting measurement data to a mathematical model chosen to fit the magnetic contour and the spatial density of data measurement positions. This precludes real-time or near real-time determination of source location. PA1 f. The localization accuracy is dependent upon the orientation of the generating neuronal source relative to the MEG sensor axis. PA1 g. Localization algorithms used to fit theory to the contour map data matrix are computationally burdensome.